The invention relates to a cold cathode fluorescent lamp (CCFL) circuit and, more particularly, to devices for rapidly lighting a CCFL.
CCFL is a widely used light source in electronic devices such as scanners, LCD panels, notebook PCs and LCD televisions. Illumination of a conventional CCFL, however, may take up to 3 minutes to achieve stability after power up. FIG. 1 is a block diagram of a conventional electronic device 10 comprising a CCFL circuit. Electronic device 10 comprises a power supply 11 and a CCFL circuit 12 comprising at least one CCFL 121. Power supply 11 provides a voltage DC1 to the CCFL circuit 12. CCFL circuit 12 generates a high voltage AC supply according to the voltage DC1 to light CCFL 121.
A method to drive a CCFL is provided by Johnson, et al. (U.S. Pat. No. 5,907,742, “Lamp control scheme for rapid warm-up of fluorescent lamp in office equipment”). The lamp is over-driven with high current for up to a predetermined time limit to accelerate mercury vaporization. The drive current is then reduced to a normal level. Between each use, the lower lamp current is 40% of the normal level to keep the lamp warm while extending the life of the product.
Using Johnson's Method, a certain amount of power is required when the CCFL is not working to maintain the temperature of the CCFL. The power consumed at the lower lamp current state is significant and additional logic circuits are required for controlling current in different states. Furthermore, if a longer length CCFL is utilized in electronic device 10, to raise the current when lighting the CCFLs, the CCFL circuit 12 receives higher voltage DC1 and generates higher voltage AC power. Consequently, transformers used in CCFL circuit 12 may not be capable of sustaining such a high voltage and may need to be upgraded, which leads to additional manufacturing cost.
A driving device for a CCFL circuit comprising at least one CCFL is provided. The driving device comprises: at least one heating device controlling the temperature of the CCFL; a power supply outputting a first voltage; a switching device having a first input terminal receiving the first voltage and a first and second output terminal outputting a second and third voltage respectively. The first output terminal is coupled to the CCFL circuit to output the second voltage when the first voltage is higher than a first level. The second output terminal is coupled to the heating device to output the third voltage when the first voltage is lower than the first level. The driving device further comprises a controller controlling the power supply to output the first voltage higher than the first level to light the CCFL, and lower than the first level to control the temperature of the CCFL.